gcoap: fix underflow when correcting ETag from cache [backport 2023.01] #40880

	name: check-pr
	on:
	pull_request:
	types: [opened, reopened, labeled, unlabeled, synchronize]
	pull_request_review:
	types: [submitted, dismissed]
	jobs:
	check-labels:
	runs-on: ubuntu-latest
	steps:
	- uses: RIOT-OS/[email protected]
	with:
	access_token: ${{ secrets.GITHUB_TOKEN }}
	unset_labels: 'CI: needs squashing,State: waiting for CI update,State: waiting for other PR,Process: blocked by feature freeze'
	cond_labels: '(Process: needs >1 ACK,review.approvals>1),(Area: RDM,review.approvals>2)'
	check-commits:
	runs-on: ubuntu-latest
	if: ${{ github.base_ref }}
	strategy:
	fail-fast: false
	matrix:
	check: [commit-msg, pr_check]
	steps:
	- uses: actions/checkout@main
	with:
	# Use the SHA of the PR branch as-is, not the PR branch merged
	# in master (default behavior in GH actions)
	# See https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit
	ref: ${{ github.event.pull_request.head.sha }}
	fetch-depth: 0
	- name: Fetch base branch
	run:
	git fetch origin '${{ github.base_ref }}:${{ github.base_ref }}'
	- name: Run checks
	run: \|
	./dist/tools/${{ matrix.check }}/check.sh "${{ github.base_ref }}"

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